PGI.sub.2 is a natural physiologically active substance having the following structural formula, which is biosynthesized from PGH.sub.2 in the metabolic process in vivo called arachidonate cascade. ##STR10## (Nature, 263, 663(1976), Prostaglandins, 12, 685(1976), ibid, 12, 915(1976), ibid, 13, 375(1977) and chemical and Engineering News, Dec. 20, 17(1976))
PGI.sub.2 have been confirmed to possess very strong inhibitory action on human blood platelet aggregation and adhesion, inhibitory activity of gastric acid secretion, vasodilatating activity, etc. Therefore it has been considered that PGI.sub.2 is useful for prevention and/or treatment for thrombosis, arteriosclerosis, ischemic heart disease, gastric ulcer, hypertension etc. But its application for pharmaceuticals is limited because of its chemical unstableness and difficulty of separation of the actions according to purpose. Accordingly, various PGI.sub.2 derivatives were synthesized and many researches have been carried out for keeping of continuity and separation of the actions. But, now we have no satisfied results yet. So, recent research goes to find PGI.sub.2 receptor agonist which have non-PG skelton to solve two above problems.